Professional Forecaster experience since 1977, concentrating in Aviation, Tropical and Long Range forecasting.
By: Steve Gregory , 09:03 PM GMT am 14. Februar 2017
TUESDAY: 14-FEB-17 / 3:00 PM CST
UNUSUAL WARMTH & STRONG STORM SYSTEMS AHEAD
INCREASING CHANCES FOR COLDER WX IN EARLY MARCH
The highly progressive flow that has prevailed this winter - in combination with an especially strong large-scale mean TROF over the EPAC - will continue to bring strong short wave TROFS /storm systems into the southwestern US every few days while simultaneously building a strong ridge over the central and then eastern US. This general pattern is expected to persist through the end of the month before a possible pattern shift evolves in early MAR.
After each strong short wave TROF /storm system comes onshore (with the potential for more flash flood and mud slide risks in CA), they are expected to move eastward towards the Gulf coast states – staying south of the upper level ridge expected to be in place over the center of the nation. While it is too early to say with any certainty that severe T-storms / tornadoes will occur – the expected pattern appears likely to support 2 or more severe WX outbreaks across the Gulf coast states during the remainder of the month.
A major driving force of the current pattern is a strong MJO now moving towards the central equatorial Pacific and has been destructively interfering with the low frequency atmospheric base state that has prevailed this winter. As the MJO moves into Phase 1 & 2, associated tropical forcing onto the mid-latitude flow pattern across the Pacific will weaken, with model trends now suggesting the development of a ridge in the west and a TROF (and more seasonal Temps) in the east by early MAR.
ENSO TRANSITION IN PROGRESS
Officially, La Niña has ended as SST anomalies within the ENSO region are back near normal. However, other atmospheric indicators continue to be characteristic of La Nina. That said, with the passage of the strong MJO across the Pacific (the strongest since last spring), a significant oceanic Kelvin Wave has been triggered. The KW is already leading to warming of SST’s in the WPAC - and this warming is forecast to advance into the EPAC over the next month. With this development – along supporting climate model forecasts - the development of El Niño conditions later this year is becoming a distinct possibility.
Fig 1: PAST 30 & 90 TEMP ANOMALIES Temp anomalies during the past 30 days have averaged 8˚F or more above normal east of the Rockies, with only the PAC NW remaining below normal. This is similar to the 90-Day Temp anomaly pattern, with Temps averaging 3˚-5˚F above normal since mid NOV – rather amazing since this period includes nearly 4 weeks of well below normal Temps in the eastern half of the nation.
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Fig 2: TELECONNECTION FORECASTS (GFS ENSEMBLE) A warmer than normal Temp pattern is supported by all 3 Teleconnections as the AO and NAO move into a positive phase while the PNA shifts back towards neutral. (Some models, not shown, actually suggest a negative PNA phase will develop by next week – and this is very supportive of warmer than normal conditions thru the end of the month.)
Fig 3: GLOBAL MODEL FORECASTS FOR THE MJO After nearly 6 months of very weak MJO cycles – we have a very robust MJO moving thru Phase 8 and will eventually move into Phase 1 / 2 as it weakens during Week 2. Longer range, ECMWF based forecast products suggest another significant MJO will develop in the Indian Ocean during March.
Fig 4: OUTGOING LONG-WAVE RADIATION (OLR) Even though SST’s have now returned to near normal in the tropical EPAC, several atmospheric metrics, including the above satellite derived OLR showing below normal cloudiness near the Dateline - remain indicative of a La Niña base state. However, with the MJO now moving towards the central Pacific, above normal cloudiness and convection is likely to develop in the central and then eastern Pacific during the week ahead.
Fig 5: UPPER OCEAN HEAT ANOMALIES ACROSS EQUATORIAL PACIFIC The below normal oceanic heat content in the equatorial Pacific that developed last summer is now rapidly weakening, with significant warming over the WPAC. It also appears that the MJO has triggered a significant Kelvin Wave in the WPAC; and this KW is seen propagating across the Pacific during the spring. This in turn should lead to warming SST’s – raising the prospect for the development of an El Niño condition during the summer or fall season - especially if we see additional, strong MJO’s and potential Westerly Wind Bursts (WWB).
Fig 6: CFSv2 ENSO FORECAST The latest CFS forecast for a sharp rise in SST anomalies within the ENSO 3.4 Region during the next 4 weeks – and this is a reasonable expectation as a result of the current MJO cycle strength. There is considerable uncertainty in the longer ranges – and this is typical of model forecasts during the spring season. Still, the prospect for an El Nino going into next winter is a real one.
Fig 7a/b: Operational GFS 500mb (≈ 18,000 Ft) Height Forecast Loop for the next 2 Weeks (Top) and corresponding 250mb ( ≈35,000 Ft) Wind Forecast Loop (Bottom) The highly progressive flow continues thru the period with the mean, synoptic scale TROF in the EPAC seen retrograding towards the west by the end of the month while a ridge begins to develop in the west. Until then, however, frequent storm/frontal systems are expected to move into the SW US and then heading towards the Gulf coast region … The very strong east-west orientated polar jet stream over the Pacific will continue to send strong short wave / storm systems into the SW US, with periodic intensification of the sub-tropical jet stream ahead of each system as they move onshore. This may result in torrential rainfall events over California during the next 10 days. As the systems reach the Gulf states region, the strong jet streams combined with increased surface (and moisture) convergence will raise the prospect of severe T-storm and tornadic activity.
Fig 8: CFSv2- TEMP ANOMALY FORECAST The latest CFS (Climate Model) forecast for MARCH suggests the general anomaly pattern observed this winter will continue during MAR and is supported by OPNL and Ensemble model trends. While the CFS model does not have a ‘stellar’ record, it has shown some skill over the past 3 years relative to pure chance. This forecast is also supported by the NMME ensemble climate model (not shown).
Fig 9: 2016 High Temporal Resolution Surface Height (SSH) Anomalies from the JASON (NASA) Satellite platform highlighting since AUG 1, 2016: The above looped imagery is assembled EVERY day (the highest temporal resolution available), showing the evolution out of last years’ El Niño into the ‘weak’ La Nina that is now gone. (As discussed elsewhere, SST’s alone do NOT fully characterize an ENSO event.) Warmer water occupies a greater volume than cold water, so higher than normal Sea Surface Heights (SSH) depicted by the warmer color tones correlate with warmer than normal ocean Temps – while colder ocean waters correlate with lower than normal SSH’s. Keep in mind that actual SST anomalies do NOT always correspond on a one-to-one basis with Total Ocean Heat Content (OHC) of the corresponding column of water , which is generally represented by the JASON SSH anomalies. As La Niña developed during the Fall, SSH’s in the ENSO region fell off to slightly below normal levels even as above normal readings persisted to the north of the ENSO region proper, while a strip of cooler waters can be seen further north. The warmer waters likely contributed to the development of especially strong Precip events in CA. Of note, is the effective demise of sub-surface warmth in the Gulf of Alaska (GOA) region which some believe had led to the ‘ridiculously resilient ridge’ in the West during the 2012-2015 winter seasons.
Fig 10: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from today’s 12Z operational GFS model run (with only minor adjustments towards the raw model data points for Days 6-8). Warmer than normal Temp anomalies will dominate the nation during Week 1 – with extremely warm anomalies over the central US. The only real exception will be in CA where at least 2 distinct storm systems will keep daytime Temps near normal. Overall confidence is very high with excellent temporal and regional model agreement – with a reading of ‘5’ for the pattern and ‘4’ for actual anomaly magnitudes on a scale of 1 to 5.
Fig 11: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (5%), ECMWF Ensemble (5%) and Climatology (10%) using the projected pattern along with explicit surface and 850mb (~5,000 FT) Temp forecasts. Some Temps are adjusted for known or expected anomalous thermal patterns & projected storm system passages.ON AVERAGE, warmer than normal Temp anomalies should persist east of the Rockies during Week 2. However, daily anomalies will be declining during the second half of the week. Overall confidence is near average for the anomaly pattern but a bit below normal for this time of year due significant differences between model runs for Days 11-16 with a reading of ‘3’ for the pattern but only ‘2’ for actual anomaly magnitudes on a scale of 1 to 5.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.